1. Field of the Invention
The present invention relates generally to the field of fuel cells and, more specifically, to a fuel container and delivery system for a liquid feed direct oxidation fuel cell.
2. Background Information
Fuel cells are devices in which an electrochemical reaction is used to generate electricity. A variety of materials may be suited for use as a fuel depending upon the materials chosen for the components of the cell. Organic materials, such as methanol or formaldehyde, are attractive choices for fuel due to the their high specific energy.
Fuel systems may be divided into xe2x80x9creformer-basedxe2x80x9d(i.e., those in which the fuel is processed in some fashion before it is introduced into the cell) or xe2x80x9cdirect oxidationxe2x80x9d in is which the fuel is fed directly into the cell without internal processing. Most currently available fuel cells are of the reformer-based type, but field-processing requirements for such cells limits the applicability of those cells to relatively large systems.
Direct oxidation fuel cell systems may be better suited for a number of applications such as smaller mobile devices (i.e., mobile phones, handheld and laptop computers), as well as in larger applications. One example of a direct oxidation system is the direct methanol fuel cell system or DMFC. In a DMFC, the electrochemical reaction at the anode is a conversion of methanol and water to C02, H+ and exe2x88x92. More specifically, a liquid hydrocarbon solution (typically aqueous methanol) is applied to a protonically-conductive (but, electrically non-conductive) membrane directly using a catalyst on the membrane surface to enable direct oxidation of the hydrocarbon on the anode. The hydrogen protons are separated from the electrons and the protons pass through the membrane electrolyte, which is impermeable to the electrons. The electrons thus seek a different path to reunite with the protons and travel through a load, providing electrical power.
The carbon dioxide, which is essentially a waste product, is separated from the remaining methanol fuel before such fuel is re-circulated. In an alternative usage of the carbon dioxide this gas can be used to passively pump liquid methanol into the feed fuel cell. This is disclosed in U.S. patent application Ser. No. 09/717,754, filed on Dec. 8, 2000, for a PASSIVELY PUMPED LIQUID FEED FUEL CELL SYSTEM, which is commonly owned by the assignee of the present invention, and which is incorporated by reference herein in its entirety .
The methanol fuel cell has been the subject of intensified recent development because of its high energy density in generating electric power from fuel. This has many benefits in terms of both operating costs and environmental concerns. Adaptation of such cells to mobile uses, however, is not straightforward because of technical difficulties associated with reforming the hydrocarbon fuel in a simple and cost effective manner. Further, a safe and efficient storage means for the hydrogen gas presents a challenge because hydrogen gas must be stored at high pressure and at cryogenic temperatures or in heavy absorption matrixes in order to achieve useful energy densities. It has been found, however, that a compact means for storing hydrogen is in a hydrogen rich compound with relatively weak chemical bonds, such as methanol (and to a lesser extent, ethanol, propane, butane and other hydrocarbons). Thus, the DMFC has been developed.
Depending upon the application with which the DMFC is ultimately employed, it may be desirable that the cell operate efficiently regardless of physical orientation. As such, the fuel delivery system that supplies the fuel to the cell should be capable of delivering the fuel in a variety of orientations, and independent of the volume of liquid in the system.
In addition, the components (including the management systems in the DMFC) are minute and subject to clogging if the fuel is in poor, impure or contaminated condition. Components on the anode side of the fuel cell need to remain uncontaminated for proper operation of the cell. Therefore, a very pure fuel must be fed to the anode in order to provide optimal DMFC performance.
While methanol has many attractive qualities as a fuel, it can be hazardous in certain proportions. In a pure state methanol is colorless and its vapor is virtually odorless. As such, there are numerous regulations and guidelines directing that certain additives be introduced to methanol that is used in commercial products (such as windshield washer fluid used in automobiles). These additives provide certain characteristics such as creating an unpleasant taste, adding smell to methanol vapors, as well as adding color to flame. It is possible, however, that some of these additives would react with an anode catalyst in a direct methanol fuel cell, or have a detrimental effect on the membrane. For example, some of the additives may adhere to the anode, blocking an active portion of the catalyst or otherwise impeding the performance of the anode, and therefore the DMFC.
It is thus an object of the present invention to provide a fuel storage container and delivery system for a direct oxidation fuel cell that introduces xe2x80x9ccleanxe2x80x9d fuel in the form of either pure fuel or an aqueous fuel solution to the cell. It is a further object of the invention to provide a delivery system which allows for mixture of additives with the fuel prior to release of the fuel into the environment outside of the system. There remains a need, therefore, for a fuel storage container and delivery system that delivers pure fuel or an aqueous solution to the system while providing the capability of mixing the fuel with additives prior to its release elsewhere.
It is a further object of the invention to provide a delivery system that continues to supply fuel solution to the system, even while the system is in various orientations.
The present invention provides a delivery system in a protective multiple-walled fuel assembly for use with a direct oxidation fuel cell system. In one embodiment of the invention, the assembly includes an outer, firm-walled container. An inner tank is disposed entirely within the outer container. A plenum is defined between the outer container and the inner tank. The inner tank holds the fuel, which may be pure fuel or an aqueous fuel solution. The plenum is filled with one or more additive substances which, when mixed with the fuel, provide color, taste, and odor to enhance recognizability of the fuel.
The delivery system includes a needle such as a hypodermic needle having a hollow central portion. The needle is introduced through an opening in the outer container and pierces the inner tank to draw fuel out of the inner tank for delivery to the direct oxidation fuel cell. The needle withdraws the fuel either under the force of gravity or under pressure provided by a pressure chamber, which can also be disposed within the outer container. A pump may be used to provide suction to draw fuel from the delivery system.
The assembly thus delivers the fuel to the direct oxidation fuel cell system. However, should the cell or the container be dropped or subjected to force that ruptures the assembly, both the inner tank containing the fuel and the outer container (including the additives) are ruptured. As a further precaution, a coupling agent, which may be a wire or string, can be employed between the inner tank and the outer container to cause rupture or induce rupture of the inner tank in the case that the outer tank ruptures first. Accordingly, this causes mixing of the fuel with the additives, thus providing the safety features of a liquid fuel mixed with additives.
In accordance with another aspect of the invention, the inner tank is a flexible bladder that is filled with fuel. Again, there is a plenum between the outer container and the flexible bladder which plenum contains the additives. There is also a pressure chamber disposed within the container, which applies pressure to the bladder. The outer container is sealed, and a needle is introduced through the seal to draw out the fuel. The bladder deflates under the applied pressure as the fuel is consumed by the fuel cell. This embodiment exhibits enhanced performance because the shape of the flexible bladder conforms generally to the volume of the liquid it contains. Accordingly, the liquid fuel is accessible to the fuel cell independent of the volume of liquid in the bladder or of the orientation of the assembly.
In accordance with yet a further aspect of the invention, a safety device is provided on the needle that is introduced into the flexible bladder that serves as the inner tank in this embodiment. When the needle is withdrawn from the flexible bladder, the safety device causes a tear in the bladder that causes the fuel to mix with the additives in the plenum. Accordingly, a safer liquid is provided that can later be more easily disposed of before refilling of the fuel delivery assembly, or disposed of directly in a single usage embodiment.